This invention relates to pigmentary bright primrose yellow monoclinic bismuth vanadate and processes for the preparation thereof.
A pigment is an insoluble small particle solid which is incorporated in paints, plastics, and inks to impart color and/or opacity. Ideally, a pigment should have strength, that is, a low color pigment requirement when mixed with a white pigment to make tints; intensity, that is, purity of shade or absence of dullness or greyness; lightfastness, that is, resistance to color change when exposed to sunlight in a pigmented article; and resistance to bleed, that is, low migration of color in a pigmented article. In addition, for many applications, it is desirable that a pigment have good hiding power, that is, ability to efficiently opacify a pigmented article.
The major intense yellow pigments currently used are lead chromate, cadmium sulfide, nickel titanate, and a number of organic materials such as certain azo compounds. Lead chromate and cadmium sulfide have good hiding; however, at present there is a trend away from using these compounds as pigments because of their possible toxicity. Nickel titanate has good hiding power but it does not have the strength and high intensity desired in a high-performance pigment. The organic pigments generally exhibit high strength but poor hiding and frequently poor bleed resistance.
Thus, there is a need for a nontoxic, high hiding, yellow pigment which also has good strength, high intensity, good lightfastness, and no bleed in organic solvents or vehicles.
Bismuth vanadate occurs in nature in an orthorhombic form commonly called pucherite. Pucherite is a dull yellow-brown mineral, which is not useful as a pigment. Pucherite is similar in color to limonite, a dull yellow-brown iron oxide, which has never been produced as a bright, high intensity pigment despite repeated attempts.
Various references have described the synthetic preparation of bismuth vanadate, for example, I. M. Gottlieb and C. R. Rowe, "Preparation and Thermal Properties of Bismuth Orthovanadate," Thermal Analysis, Vol. 2, Proceedings Third ICTA DAVOS (1971), pp. 303-311; R. S. Roth and J. W. Waring, "Synthesis and Stability of Bismutotantalite, Stibiotantalite and Chemically Similar ABO.sub.4 Compounds," The American Mineralogist, Vol. 48 (Nov.-Dec., 1963), pp. 1348-56; H. E. Swanson et al., "STandard X-Ray Diffraction Powder Patterns," National Bureau of Standards Report, No. 7592 (Aug., 1962); Eduard Zintl and Ludwig Vanino, "Process For The Manufacture Of Pure Bismuth Vanadate"; German Pat. No. 422,947 (1925). However, as in the case of yellow iron oxide, none of the above were successful in preparing bismuth vanadate as a bright yellow pigment.